Different Types of Wind Turbine

Wind turbines are classified in two ways. Firstly, the classification depends on the position of the rotor axis of wind power system and secondly, it depends on the variation of wind speed. Wind turbine can be classified into horizontal axis wind turbine (HAWT) and vertical axis wind turbine (VAWT) [15]. Depending on the variation of wind speed wind turbines are also classified into constant speed wind turbine and variable speed wind turbine [16].

HAWT: Most popular commercial wind turbine connected to a grid, in recent times is the HAWT. These mainly consist of two or three bladed rotors. For this type of wind

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turbine, the rotor shaft and electrical generator are placed at the top of the tower where wind is less turbulent and wind has more power. Figure 1.7 (a) shows the schematic diagram of HAWT [15].

Figure 1.7: (a) Schematic diagram of horizontal axis wind turbine. (b) Schematic diagram of vertical axis wind turbine [15]

VAWT: The main rotor shaft in VAWT is placed vertically in the wind turbine system. Figure 1.7 (b) shows Darrieus rotor type vertical axis wind turbine system which is the most popular type VAWT [15]. The main advantages of this kind of wind turbines are that, the generators and gearboxes are easy to repair and perform maintenance operation of the components as they are placed close to ground, and also this turbine can catch the wind from all direction without any yaw system. Though there are advantages, but this type of wind turbine has many disadvantages like, low starting torque, sensible to design conditions, tendency to stall under blustery wind conditions, used for low power

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applications such as battery charging, as output power is very low; thus, minimizes the popularity of VAWT comparing with HAWT [17].

Constant Speed Wind Turbine: This type of turbine is simple in design and is composed of a gearbox, a low speed shaft, a high speed shaft and an asynchronous generator. Figure 1.8 presents the schematic diagram of a constant speed wind turbine system. Mainly squirrel-cage induction generator is used to generate power from such type of system [13]. For this turbine system, a power-electronic converter is not needed since the system works at constant speed, and therefore it is structurally simple. Synchronous frequency is imposed by the grid to the machine because the generator is directly interfaced with the utility grid through a turbine transformer machine. The rotational speed of the asynchronous generator is not exactly constant and varies within ±3% to ±8% of the synchronous speed which is very small, so this type of turbine is considered to be a constant speed type wind turbine system. As asynchronous machines consume reactive power, so power factor correction capacitors is used at each wind turbine to maintain the voltage profile to get stable operation, mainly at non-stiff grid conditions. Though simple in design, this type of turbine system is not able to extract maximum power at different wind speed as it works at almost constant wind speed. The stator of generator is directly connected to a utility grid, so, as a result of fact, any kind of transmission line network will affect directly the wind based generating units of the system [16].

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Gearbox

Machine

Figure 1.8: Schematic diagram of constant speed wind turbine system [16]

Variable Speed Wind Turbine: Though structurally complex, this type of wind turbine system is more popular comparing with constant speed wind turbine as maximum energy can be generated due to the variability in speed of the turbine system. A solid-state power converter is always used to interface the machine to the transmission system. There are two different types of variable speed wind turbines that are most common nowadays. These are: wind turbine with doubly fed induction generator (DFIG) and wind turbine with fully rated converter (FRC) based on a synchronous or induction generator [13].

• DFIG Wind Turbine: Power can be delivered to the grid through both stator and rotor of the machine for this type of variable speed wind turbine system. Depending on the rotational speed of the generator the rotor can also absorb power [13]. The rotor absorbs power from the grid through power converter when the operating speed of generator is lower than the synchronous speed and it delivers power to the grid when the generator operates above the synchronous speed [13]. A schematic diagram of this type of turbine is shown in Figure 1.9 (a). A controllable voltage is injected into the rotor at slip frequency to achieve variable speed of operation. Mainly two

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AC/DC IGBT based voltage source converters (VSCs), linked by a DC bus is interfaced in between the rotor winding and electrical network to decouple transmission line electrical frequency from rotor mechanical frequency. Thus, the variable speed operation of wind turbine is possible [13].

• FRC Wind Turbine: Figure 1.9 (b) shows a schematic diagram of FRC wind turbine system. A wide range of electrical generators like wound rotor synchronous generator, permanent magnet generator, induction generator etc. can be used for this type of turbine system. Use of the gearbox is optional for FRC wind turbine system. Variable speed operation can be performed as the dynamic operation of electrical generator is effectively separated from the electrical network through power converters [13]. A diode rectifier or a PWM VSC can be used as generator side converter and only PWM VSC can be used as grid side converter. Each type of converter can independently deliver or absorb reactive power independently. Mainly DC bud voltage is controlled by the grid side converter whereas generator side converter controls the torque applied to the generator. But reverse control strategy is also applicable for these two types of converters [13].

20 Gearbox Machine AC/DC/AC Power Electronic Converter (a) Gearbox

Machine AC/DC/AC Power

Electronic Converter

(b)

Figure 1.9: (a) Schematic diagram of DFIG variable speed wind turbine system [16] (b) Schematic diagram of FRC variable speed wind turbine system [16]